Dishonored is the first game from Arkane since it was acquired by Bethesda, and the game is being developed with the help of the lead designers of Deus Ex and Arx Fatalis.

Overview

You are the once-trusted bodyguard of the beloved Empress. Framed for her murder, you become an infamous assassin, known only by the disturbing mask that has become your calling card. In a time of uncertainty, when the city is being besieged by plague and ruled by an oppressive government armed with neo-industrial technologies, dark forces conspire to bestow upon you, abilities beyond those of any common man – but at what cost? The truth behind your betrayal is as murky as the waters surrounding the city, and the life you once had is gone forever.

This game reminds me of Half-Life 2 and BioShock. I'm sure the PC version won't be a poorly optimized console port. The developers, Arkane Studios, are the same guys that made Dark Messiah of Might and Magic. It was made with the Source engine and during the game's development, they worked on a Half-Life game called "Return to Ravenholm". It was cancelled but it shows that they had a working relationship with Valve (which is primarily a PC game developer). I don't know what their experience is with Unreal Engine 3 but judging by the screenshots and videos, I'd say they're quite talented. From what I've read online so far, the game is using a custom version of Unreal Engine 3 with a lot of extra stuff developed in-house.